The Saturnian System

(this is effectively a poster, if you want to download it, but it uses a lot of black ink).

Saturn and its moons are the second example of a mini-solar system within the big one. For thousands of years, Saturn was thought to be the outer limit of the Solar System, and has its own associations because of that, but for today I want to concentrate on the whole system of Saturn, with moons, rings and magnetosphere all included, rather than the planet itself.

Saturn has a prodigious number of moons, the count sometimes exceeding Jupiter’s. This is because of the Titius-Bode series. As you go further out, the orbits of the planets get more widely separated, meaning that a planet of the same mass has a longer gravitational reach over its surroundings. Saturn is of course considerably less massive than Jupiter, but its Hill Sphere, the region where its gravity is dominant, is bigger than Jupiter’s, at 1025 radii compared to Jupiter’s 687. Working this out in kilometres, Jupiter’s has a diameter of 96 million kilometres and Saturn’s is 119 million. Against this is the fact that the system is less cluttered out by Saturn than it is near Jupiter, with the asteroid belt being near the larger planet. Saturn has eighty-three moons not including the ones which form part of the rings, compared to Jupiter’s eighty. There was a point when Saturn’s moon count far exceeded Jupiter’s, but this seems to be over. The Hill Spheres are nowhere near each other and there is no competition between the two in this way. Unlike the magnetospheres.

When Voyager 2 was on its way to Saturn, it encountered Jupiter’s magnetotail in February 1981, which may indicate that the tail is forked. It did so again in May that year by which time it was nine-tenths of the way there, or around eighty million kilometres from Saturn. Saturn can even be within Jupiter’s magnetotail at times. As far as Saturn’s magnetosphere is concerned, all its moons out to Titan orbit entirely within it. Titan itself is very close to the edge and passes in and out of it, spending about a fifth of its time within. It’s surrounded by a doughnut of hydrogen extending inwards to Rhea, which is the second-largest moon. The bow shock is somewhat further out and extends north and south of the planet for at least thirty radii. Sunward it extends for almost two million kilometres. This means that of the large moons, only Iapetus and Phoebe orbit outside it entirely. As well as the neutral hydrogen torus around the orbit of Titan, there’s an inner torus of rarefied plasma of ionised hydrogen and oxygen, which effectively means protons and oxygen ions, whose outer diameter is about 400 000 kilometres. At the edge of this torus the temperature is over 400 million degrees C, but it should be born in mind that Earth’s thermosphere is 2 500°C and the Sun’s atmosphere is over a million Kelvin, which is hot but didn’t destroy the probe recently sent there. Temperature really represents the average kinetic energy of the particles and not heat. In a sauna, the air temperature can be over 100°C but the effect on the human body is nowhere near as harsh as boiling water for this reason.

Titan comprises 96% of the mass of all Saturn’s moons put together. This seems actually to be more typical than Jupiter with its four large moons, as similar mass distributions are found among the moons of Uranus and Neptune. The whole system has a kind of quietness and serenity to it, at least from afar. Some of the moons are active, but there’s nothing like the hot volcanism found on Io. All the moons are substantially icy. Saturn’s moons are unique in that some of them have trojans – moons which share their orbits but are sixty degrees behind or ahead of the larger moons. Saturn in general has quite a cluttered and ice-strewn neighbourhood in connection with its rings, and this seems to be part of this aspect of it. This means that the exact number of moons can never be determined because the size of bodies orbiting it goes all the way down, fairly evenly, to miscroscopic grains of ice and dust. In a way, all that can be said is that Titan is the biggest by far, being about the same size as Ganymede.

The five large inner moons, Mimas, Enceladus, Tethys, Dione and Rhea, all participate in the magnetosphere, absorbing protons, as do the particles making up the very sparse E ring. I’ll talk about the rings in detail when I get to Saturn itself, but another unique feature of Saturn’s system is the interaction between the particularly substantial rings and the magnetosphere. The other giant planets have much less substantial rings and therefore less significant interactions. Electrons are absorbed by the main rings, and below the main rings towards Saturn is the least radioactive region of the entire Solar System outside of large bodies and their atmospheres because the rings act as a radiation shield. There is, however, nothing as strong as the plasma tunnels and torus around Io, which influences radio transmissions from Jupiter.

Radio signals from Saturn are weaker than the ones from Jupiter in a broad range from twenty kilohertz to one megahertz, so listening to long or medium wave radio stations there would be right out. Like Jupiter’s System III, which is the common rotation of the interior of the planet with its magnetosphere, Saturn has its own System III, lasting ten hours, 34 minutes and two dozen seconds. There is nothing as strong as Io’s influence, but there is a relatively mild variation corresponding to the time taken for Dione to orbit, 2.7 days. This could be coincidence. When Saturn passes close to Jupiter’s magnetotail, the radio transmissions become undetectable but it isn’t clear whether they cease because of it or are just overwhelmed by Jovian radio noise.

The moons have fairly regularly spaced orbits out to Rhea, although there are some smaller moons which either share orbits with larger moons or regularly swap over. Titan, though, is over twice as far from Saturn as Rhea, then Hyperion is relatively close to Titan, Iapetus over twice as far from Saturn as Hyperion, and finally Phœbe is much further out and orbits backwards compared to the others and the majority of other worlds in the Solar System. This suggests that Phœbe is a captured asteroid. Surprisingly, although it was discovered in 1898, no moons further out were found until the twenty-first century despite the fact that the planet was visited several times by spacecraft. However, almost four dozen moons have now been found which orbit backwards. More than two dozen moons have yet to receive names because there are just so many of them. Even the most distant moon is well within Saturn’s Hill sphere, so it’s still possible that there are more. There’s also a cluster of moons, including shepherd moons and coörbitals, near the rings and possibly even within them, but it should be borne in mind that there’s a judgement call here regarding how big a ring particle is before it counts as a moon or moonlet.

Saturn, and therefore its system to some extent, is tilted 27° with respect to its orbit. This also tilts some of the moons but others are already at odd angles and it’s fairly meaningless to regard them as influenced by this tilt. For Dermott’s Law, mentioned in connection with the Galileans a couple of days ago, T=0.462 days and C=1.59.

I’m going to end on a personal note. I don’t remember Kepler’s third law of planetary motion very clearly, so I always use Saturn to work it out. Saturn is about ten AU from the Sun, i.e. ten times Earth’s distance. The cube of this is a thousand, and that’s square root is thirty. Saturn takes thirty years to orbit the Sun once, hence the Saturn Return of astrology, meaning that the cube of the semimajor axis (average distance from the Sun) of a planet is directly proportional to the square of its sidereal period (“year”).

Next time I’ll be looking at Saturn itself, including its rings, the famous hexagon and the unexpected connection with a certain comedian.

Planetary Chauvinism

“Chauvinism” is quite an old-fashioned word for prejudice against a particular group. Nowadays each has its own word, generally consisting of the name of the type of group plus “-ism”. It comes from a Bonapartist soldier called Nicolas Chauvin, who insisted on maintaining his support for Napoleon after the Bourbon Restoration, and was then extended to apply to any type of fanatical devotion to or against a group or cause. In the light of the dangers posed by the use of the word “terrorism”, it might be worth bringing it out of retirement to refer to a particular kind of fanaticism which doesn’t currently have an obvious word to describe it, although “fanatic” is a less ostentatious option.

The use of “male chauvinist pig” apparently dates back to the 1930s CE. It has a rather old-fashioned tone to it now, but maybe it deserves reviving. For a start, it doesn’t lend itself to referring to sexism both ways, which is a contentious issue. It can only mean prejudice against women and girls. “Female chauvinism” is also used sometimes. A notable aspect of it is that it refers to the individual in the group to which there is a bias rather than a group, one member of which there’s a bias against. “Racism”, for example, refers to the category of race and not to a specific ethnicity, but very often refers to White racism against others, and this centring on the member of the group responsible for the prejudice is quite helpful conceptually. I don’t think “White chauvinism” is a common utterance, although there’s an interesting Communist pamphlet with that title dating from 1949, but it works quite well as a way of emphasising Whiteness and White fragility. However, the word has long since gone out of fashion in these uses.

A more specific use of the word “chauvinism” seems to have started with the well-known science populariser Carl Sagan in the late 1960s. He uses it to refer to biasses in ideas about extraterrestrial life. Examples would be “carbon chauvinism” and “water chauvinism”. The idea here is that a particular characteristic of life as we know it on this planet leads us to conclude that all life must have that characteristic, and this restricts the places and circumstances in which we might consider or look for other kinds of life. It might even affect how we view life on this planet because of the possibility of a “shadow biosphere”. It’s conceivable that even on, or perhaps in, Earth, there are other forms of life which don’t share our chirality or chemistry. For instance, the phenomenon of desert varnish, a dark coating which forms on rocks in arid areas, has been suggested as the action of undiscovered life forms which are not like the ones we know about, and a more outré suggestion is that silicon-based organisms live within this planet but never come anywhere near the surface. Carl Sagan, if I recall correctly, described himself as a carbon chauvinist but “not that much of a water chauvinist”. That is, he couldn’t conceive of a way biochemistry could emerge if it wasn’t based on carbon, although he did believe in the possibility of other elements substituting for some of our own. Here are a few entries from his Encyclopedia Galactica:

This one appears to have carbon, hydrogen and oxygen like us but lacks nitrogen, sulphur or phosphorus. It also utilises helium, which must be non-chemical. Germanium and beryllium also have no biological rôle on this planet, and it looks like this civilisation has no historical association with planets.

More details of the same explain further. They are not a single species but an alliance of some kind, perhaps symbiotic, and can apparently only survive in interstellar space because they depend on superconductivity, which only occurs at a low temperature.

This is us:

The last entry might be a bit depressing! This was in 1980.

I mention chauvinism now because I’ve had some difficulty wording my writing in this blog recently. There is an issue with the way we can refer to what I’m going to call “worlds” for argument’s sake in this paragraph. We tend to talk about planets as potential abodes for life, including technological cultures, but this is rather misleading. Considering our own Solar System, we have one body which is established to have had life on it for æons, our own Earth, but other worlds have been considered. At the moment the candidates seem to be: the upper atmosphere of Venus; the surface and oceans of Earth (quite a strong candidate that one!); Mars; the upper atmosphere of Jupiter; the interior oceans of Europa, Ganymede and Callisto; the surface and interior ocean of Titan; the interior ocean of Enceladus. There are a couple of weaker candidates in Ceres and Pluto. That gives us four planets, two dwarf planets and five moons. Hence even in our own system the possible places for life as we know it are mainly non-planetary, and constantly referring to “planets” in other star systems as places where life might evolve or appear without technological intervention starts to sound rather prejudiced. Maybe planets tend to be less suitable than other types of world.

The reason for most of these possibilities in our Solar System is that they have internal oceans. Europa and Enceladus in particular have rather suitable ones. Ganymede, Callisto and probably Titan also have liquid interiors but they’re more like Earth’s mantle than oceans, which might make them less friendly to life as the supply of other elements than hydrogen, oxygen and perhaps nitrogen might be very limited or non-existent. The geysers on Enceladus, on the other hand, do contain organic molecules with molecular weights above two hundred daltons, which is slightly larger than glucose, so the complexity may be considerable, and this is the only place off-Earth so far where such large molecules have been detected. Another very common finding, even in places where life is very unlikely, is tholins, which are reddish tarry organic substances present on many asteroids, centaurs, Titan, Europa, Rhea, Pluto and Ceres, although it isn’t clear that tholins are responsible for the red terrain on Pluto. Tholins are like the “cousins” of organic life forms, because they’re generated by the action of radiation such as cosmic rays on simple organic compounds. They’re bound to be common on small solid planetoids and comets throughout the Galaxy, and the question arises of whether we are the black sheep of the family in that we’re the rare exception, or whether life is just what happens instead of tholins in similarly widespread conditions.

It seems moons with sub-“terranean” oceans are a likely place for life to develop provided there’s an energy source and sufficiently varied elements, along with sufficiently low salinity. That last criterion may be surprisingly hard to satisfy. The total amount of liquid water in the Solar System is many times that found in our oceans, and the proportion of water on the moons involved is also much greater than that of the oceans to Earth. The energy source may be the Sun but is more likely to be tidal forces acting on the moon from surrounding large moons or the large planet it orbits, or it may be radioactivity as it is with our planet’s interior. If intelligent life arose in these conditions, it might be blind, unable to produce fire and unaware of anything beyond its ocean, since there would be a thick layer of ice above it. That said, it might also be tempted to drill a hole in that ice to see what’s outside or perhaps follow the course of a geyser or cryovolcano out into space, and it would be easier to leave most moons’ gravity wells than Earth’s, particularly as only Titan among these has a significant atmosphere, since they’re much smaller and less dense than this planet. It’s still possible that some kind of exothermic reaction could replace fire in their technology, but they might be stuck in the stone age if they exist at all.

I’ve already talked about exotic life in neutron and ordinary stars, which are of course not planets either, and there are also “rogue planets”, which wander through interstellar space too far from any stars to become associated with them. These will have been hurled out of star systems at some point, but life could possibly still arise on or in them if there is volcanism, or in any moons of the type mentioned if they’re tidally heated. In a sense these are actually proper planets, because the word planet means “wanderer”, which is what these do rather than orbit, which is what we tend to think of planets as doing. This actually means that etymologically these aren’t planets at all. Not only is Pluto not a planet, but nor is Mercury, Jupiter or Mars. In fact Pluto is in that sense more of a planet than the others because its orbit is more erratic and probably chaotic then theirs. However, it’s a fallacy to take the original meaning of a word as gospel and base one’s arguments on that, as can be seen with the idea that homophobia is misnamed because it’s hatred rather than fear. Maybe “heterosexual chauvinism” would be a better way to describe that combined with biphobia and panphobia.

There is also the question of what a technological species or perhaps intelligent machines would do if it got into space. In the mid-1970s, a plan for a rotary space colony about a mile in diameter (it was an American project, which might explain the units) situated at the L-5 gravitational equilibrium point between Earth and Cynthia was put together, and on this idea was built the expectation that if humans did move out into space, they might not actually be very interested in settling on, for example, Mars, when tailor-made orbital environments could be devised much more easily. It’s debatable whether such habitats are economically viable and the first would depend on the existence of industry on Cynthia to work, but there are different motives for going into space such as rescuing some, and that’s a very small fraction, of the species from a major asteroid strike or some other mass extinction-type disaster, and the motives of aliens would of course be unknown. Nonetheless it makes a lot of sense to bypass planets entirely and just build wheels in space, and beyond that perhaps Dyson spheres and ringworlds. Extending this far enough into the future, perhaps the most suitable places for habitation wouldn’t be found near Sun-like stars at all but the likes of blue supergiants like Rigel or the Pleiades rather than the likes of α Centauri or τ Ceti, because the former have very deep habitable zones and plentiful radiation. These are also the names that turn up in Golden Age science fiction because people have actually heard of these places. ETs might also board space arks, initially to get to nearby stars but take so long to get there that they no longer see the point of disembarking once they reach their destinations, and just carry on voyaging. There’s another answer to the Fermi Paradox: aliens leave their home worlds, establish colonies in space or launch spaceships to nowhere (leaving any place?) and their original abodes just go wild again. Also, we’re looking at the wrong stars for technosignatures.

There is one more really wild possibility: maybe life evolves in space and stays there. Life evolving in space isn’t a particularly new idea. Fred Hoyle and Chandra Wickramasinghe claimed in 1974 that the reddening of distant galaxies attributed to the expansion of space is in fact explained by microörganisms absorbing their light and they weren the first to claim that life here comes from elsewhere. More recently it has been noted that the whole of the early Universe had the right conditions for life, being fairly warm, dense and having all the right elements in close proximity to each other, for the kind of life we know about. Cosmic strings, of course, also existed by this point, so if that kind of life exists at all, it may have done so even before that happened. This is leaving out all the other possible kinds of life, such as plasma, and there have been thoughts about life based on liquid helium or superconductors, although I don’t know how that would work in detail. All of this is very vague.

To finish then, perhaps we think too much about planets when we consider alien life. It is in fact notable that we don’t seem to have a simple word to refer to heavenly bodies which are not stars in general. Maybe if we had a future, we would find ourselves eschewing both Earth and other planets just to live permanently in space and things here could go back to how they were before we evolved. They probably will anyway after we’re extinct. Meanwhile, maybe there are countless civilisations in the Universe trapped under heavy atmospheres or the bottoms of frozen over oceans in eternal darkness who don’t even know there is anything else, while out there between the stars are wraith-like beings thousands of kilometres across with their own societies, or living starships who evolved on their own. It has been said, after all, that the Universe is stranger than we can imagine.